16 2 TEETH OF EDENTATES CHAP. inasmuch as many Edentates have teeth. It is, however, by a number of small tooth - characters that the order can be defined. Thus if teeth are present they are simple in struc- ture, without enamel in the adult condition, though a rudi- mentary enamel-organ has been discovered in an Armadillo. The teeth, moreover, are not found in the anterior part of the mouth, and they grow from persistent pulps; neither is there much differentiation among them. It is not possible, however, to speak of the Edentates as quite homodont, since in Orycteropus there are large cheek-teeth ; but there is at any rate not a marked heterodonty in that or in any other Edentate. It used to be said that the Edentates were monophyodont. But the Armadillo Tatusia was subsequently found to possess a second suppressed dentition, and after this discovery Mr. Thomas proved that Orycteropus is also diphyodont. Since then other Armadillos have been shown to be diphyodont; and the whole group there- fore, so far as concerns those members that have teeth, may in all probability be regarded as typically mammalian in this respect. These characters are slender enough, but there seem to be no others by means of which the members of this order can. be satisfactorily linked together. The fact is, that we have here a polymorphic order which contains in all probability repre- sentatives of at least two separate orders. We have at present a very few, and these perhaps highly modified, descendants of a large and diverse group of mammals. For convenience* sake they will be all treated of under the head of Edentata. Although for the probable reasons already stated it is a hard matter to frame such a definition as will include all existing Edentates, it is easy enough to define two groups in this heterogeneous order; to define one group we ^should say, rather, and then to regard the leavings as forming another not so easily definable a group. The perfectly-definable group is that which includes the American Anteaters, the Armadillos, and the Sloths. In all these creatures, which may certainly be regarded as representing on their own account as many family types, there are a number of important and highly-characteristic anatomical features which they share in common. So exceedingly different are those three types in general appearance and (correlated with that) in way of life that these common characters acquire increased importance.